This invention relates to an ebullition cooled transformer and more specifically to an ebullition cooled transformer using a condensible, liquid as an electrically insulating, cooling medium.
A conventional ebullition cooled transformer of the type referred to has comprised a core and coil assembly consisting of an iron core and a low and a high voltage coil inductively disposed around the iron core, a housing including a low portion on which the core and coil assembly is fixedly disposed by having one end of the iron core fixed to the bottom thereof, an amount of a condensible, electrically insulating, refrigerant charged into the lower portion of the housing to substantially immerse the core and coil assembly thereinto, and a cooler unit disposed on an upper portion of the housing which is filled with a vaporized portion of the refrigerant.
In operation, the core and coil assembly generates heat to boil the refrigerant. The resulting vapor is raised toward the cooler. The cooler unit exchanges heat between the vapor of the refrigerant and the outside air to condense the vapor of the refrigerant into drops of the refrigerant in the liquid phase. Thus the drops of the refrigerant fall on the refrigerant in the liquid phase disposed on the lower portion of the housing. The process as described above is repeated to continuously cool the core and coil assembly.
Also in view of the economy, the housing is not constructed to form a pressure container. In other words, the atmospheric pressure or less is the highest pressure under which the housing can be put. Thus it is a common practice to use condensible electrically insulating refrigerants having the temperature-to-pressure charactristic suitable under such a pressure or less. An example of those refrigerants involves a fluorocarbon expressed by C.sub.8 F.sub.16 O which has a vapor pressure on the order of 1 kg/cm.sup.2 abs at 100.degree. C. However, that fluorocarbon has a vapor pressure of, about 10 mmHg at 0.degree. C. That is, the vapor pressure is extremely low in a low temperature range. Also the vaporized portion of the fluorocarbon located above the core and coil assembly within the housing has a dielectric breakdown field of about 1.6 r.m.s. kV/mm which figure is low as compared with the air under one atmospheric pressure. Thus the fluorocarbon as described above is bubbled when a load is applied to the transformer put at a low temperatures. The bubbles thus formed at the low temperature are low in vapor pressure and therefore dielectric strength. Under these circumstances, therefore, it has been required to maintain a sufficient electrically insulating distance in each of the liquid and vapor phases in order to prevent the dielectric breakdown or partial electric discharges from occurring. Thus transformer of the type referred to have encountered a problem in the cancellation of the advantages that a thermal flux is large due to the heat transfer resulting from ebullition cooling and the associated coils can be made small-sized.
Accordingly it is an object of the present invention to provide a new and improved ebullition cooled transformer capable of preventing a dielectric strength from decreasing at low temperatures and of maintaining a considerable dielectric strength in the atmosphere.